Accurate and consistent shooting requires central eye alignment through a rifle scope to prevent parallax error. Parallax error causes the image of the target to move in relation to the cross hairs viewed through the scope, thus affecting accuracy. Current rifle scopes rely on the shooter's experience to align the eye correctly (i.e. on the optical axis of the scope). If the shooter misjudges this, inaccuracy can result.
Inaccuracy can also be caused by cant: that is by rotation of the rifle about its axis from the vertical plane. Unless the scope is set at zero elevation (i.e. parallel to the axis of the barrel), cant causes the shot to veer to one or other side of the aiming point.
WO 99/27408 describes a sight scope having a main reticle and a secondary reticle that is mounted within the scope and is axially displaced from the main reticle. By aligning the two reticles, correct eye alignment can be checked thereby reducing the possibility of parallax error. The scope does however have a number of disadvantages. The axial spacing of the two reticles is relatively small, which makes it difficult to see small misalignments. Because the reticles are both mounted internally and seen only in silhouette, they can be difficult to distinguish, particularly in poor lighting conditions. The secondary reticle also partially obstructs the view through the scope and may be distracting. Further, because existing scopes cannot be easily adapted to use a secondary reticle, the arrangement is applicable for practical purposes only to new scopes. The scope also does not address the problem of cant.